This invention relates to an ultrasonic inspection method and apparatus for detecting the presence or absence of defects in an object to be inspected and more particularly to an ultrasonic inspection method and apparatus suitable for examining the presence or absence of exfoliation at a bonding portion between two bodies and the presence or absence of voids in a body.
Demands for accurate and easy inspection of the presence or absence of exfoliation at a bonding portion at the interface between bodies and a void in a body have come out of various fields of technologies. Recently, these demands have been accelerated especially because a variety of electronic parts such as IC packages and various products incorporating electronic parts have been put in production and exfoliation at bonding portions and voids in bodies seriously affect performance and function of these parts and products.
In the past, as one of the methods for inspection of defects in electronic parts, an ultrasonic inspection method has been known wherein an object to be examined is immersed in a liquid vessel (typically a water vessel), an ultrasonic beam is emitted to the object from a probe also immersed in the liquid vessel, a reflecting beam from a portion of the object, for example, a bonding portion is received and converted into an RF signal, and the RF signal is displayed on a display unit to indicate the presence or absence of a defect.
In displaying RF signals on a display unit, two methods are available of which one is termed an A-scope display in which changes in amplitude of an RF signal waveform traced on the ordinate of an oscilloscope is displayed relative to the time traced on the abscissa and the other is termed a C-scope display in which a maximum value indicative of a positive peak of an RF signal waveform or a maximum in absolute value indicative of a negative peak of the RF signal waveform is produced by scanning a probe vertically and horizontally with respect to an object to be examined and is displayed in the form of a gradation display on a monitor television whose abscissa represents a moving distance of the probe in the horizontal direction (X) and ordinate represents a moving distance in the vertical direction (Y).
When an object to be inspected has a defect such as a peeled off bonding portion or a void, an ultrasonic beam is reflected approximately 100% at such a defective interface as above and the level of the reflecting beam becomes larger than that of a reflecting beam from a portion without any defect. In nature, the phase of a reflecting beam originating from an ultrasonic beam which comes into a material of a small acoustic impedance (represented by the product of density of the material and sound velocity) from a material of a large acoustic impedance undergoes inversion. Therefore, the phase of a reflecting beam from a defect such as a peeled off bonding portion or a void is inverted relative to the phase of a reflecting beam from a non-defective portion and, given the phase of the latter reflecting beam being positive, the phase of the former reflecting beam becomes negative.
Conventionally, in the A-scope display method, the inspector makes a decision empirically using the above-mentioned two natures of the ultrasonic beam as an evaluation index. In the C-scope display method, the inspector also makes a decision empirically by observing a density pattern in a resulting gradation display utilizing the former nature of the ultrasonic beam. One may refer to a relevant application, i.e., U.S. Pat. No. 4,768,155 or corresponding European Patent Application No. 86100581.7.
The aforementioned two natures of the ultrasonic beam, however, are not always clearly developed for any type of exfoliation and void. Even with the signal waveform from, for example, exfoliation, its level will sometimes increase indistinctively slightly and its phase will not sometimes be inverted distinctively. Accordingly, the conventional inspection methods which relied upon the empirical judgement by the inspector can not be well adapted for such critical events as above because the magnitude of the waveform level and/or the presence or absence of the phase inversion can not be judged correctly and even if possible, the judgement is considerably time consuming.
Further, if waveform levels from bonding portions of different examined objects are equal to each other, it is difficult to judge whether the same waveform level indicates normal bonding or exfoliation of abnormal bonding.